Study Design: This is a randomized, placebo-controlled clinical trial of 450 pregnant women who will be enrolled between 10-14 weeks of gestation and followed until delivery. Following written, informed consent, each mother will be randomized to receive either placebo or 4000 IU/day vitamin D3 plus the standard prenatal vitamin (containing 400 IU vitamin D3). She will be followed monthly for a total of nine study visits. These visits will include the initial recruitment of mother at 10-14 weeks of pregnancy, monthly study visits, delivery study visit, and 18-month post-partum follow-up visit for mother and child. The primary outcome variable is maternal and neonatal health status as a function of maternal and infant vitamin D status. Secondary outcome variables to be analyzed, as a function of maternal vitamin D status during pregnancy will include: T-lymphocyte profile, immune function indicators, neonatal growth, inflammatory cytokine profile, methylation patterns of DNA of both mother and her neonate,